It is known practice, in the cosmetics or dermatological field, to use oily compositions, i.e. compositions comprising only oils or containing very little water, for example as products for removing makeup from the skin, as massage products for caring for the face, the body, the scalp or the feet, as a conditioner balm, as an antisun oil or else as a care oil for the shower. These compositions have the advantage of being effective since the oils are directly in contact with the skin, but they have the drawback of being too liquid, and therefore not very practical for use since they run during application. In addition, they have the drawback of lacking a freshness and of leaving a greasy effect when they are applied to the skin.
The oil-in-water (O/W) emulsions consist of an oily phase (or lipophilic phase) dispersed in an aqueous phase. They therefore have an external aqueous phase and are therefore products that are more pleasant to use because of the feeling of freshness that they provide. However, they have the drawback of relatively lacking effectiveness, in particular for removing makeup or any other application where the greater the amount of oil present, the greater the effectiveness, since the oily phase or lipophilic phase that contains the oils and that constitutes the internal phase is not directly available and, as a result, the oil is less effective since the amount of oil is less. Now, for example for cleansing the face, and more especially for removing makeup, which consists in removing all makeup products, women seek to obtain as effective a makeup removal as possible.
In order to ally effectiveness and cosmetic attractiveness, it is advantageous to produce an oil-in-water (O/W) emulsion containing a large amount of oil and, in order to facilitate handling, it is advantageous for the emulsion to be in the form of a cream, which makes it possible to use it directly by taking it up with the fingers without involving a support such as cotton wool, and without fear that the product will run.
The production of O/W emulsions with a high oil concentration is often difficult, and the emulsions obtained are often unstable. These emulsions are conventionally obtained mechanically, for example by emulsification with a stator rotor or using a high-pressure homogenizer, because a great deal of mechanical energy is required to divide the dispersed phase up into small drops. In order to stabilize these emulsions, emulsifying surfactants of the oil-in-water type, i.e. with an HLB (HLB=hydrophilic lipophilic balance) ranging from 8 to 18 are generally added thereto, which emulsifiers, due to their amphiphilic structure, go to the oily phase/aqueous phase interface and thus stabilize the dispersed oil droplets. Despite the presence of the emulsifiers, the emulsions can have a tendency to destabilize (coalescence and then separation of the aqueous and oily phases with release of oil). To improve the stability of these emulsions, the concentrations of emulsifiers can be increased; however, a high concentration of emulsifiers can result in a rough, clingy or sticky feel, and also in problems of innocuousness with respect to the skin, the eyes and the scalp.
To resolve the problems of stability of conventional O/W emulsions, it has been proposed to prepare O/W emulsions obtained by the phase inversion temperature technique (PIT emulsions), in which the average size of the globules constituting the oily phase is within given limits, namely between 0.1 and 4 μm (100 to 4000 nm). The principle of phase inversion temperature (or PIT) emulsification is, in theoretical terms, well known to those skilled in the art; it was described in 1968 by K. Shinoda (J. Chem. Soc. Jpn., 1968, 89, 435). It was shown that this emulsification technique makes it possible to obtain stable fine emulsions (K. Shinoda and H. Saito, J. Colloïd Interface Sci., 1969, 30, 258). This technology was applied in cosmetics as early as 1972 by Mitsui et al. (“Application of the phase-inversion-temperature method to the emulsification of cosmetics”; T. Mitsui, Y. Machida and F. Harusawa, American. Cosmet. Perfum., 1972, 87, 33).
The principle of this technique is as follows: a mixture of an aqueous phase and an oily phase is prepared and is brought to a temperature greater than the PIT temperature, the phase inversion temperature of the system, i.e. the temperature at which the equilibrium between the hydrophilic and lipophilic properties of the emulsifier(s) used is attained; at higher temperature, i.e. greater than the phase inversion temperature (>PIT), the emulsion is of water-in-oil type and, as it cools, this emulsion inverts at the phase inversion temperature so as to become an emulsion of oil-in-water type, having beforehand passed through a state of microemulsion. This process makes it possible to readily obtain emulsions with a diameter generally less than 4 μm.
Document WO-A-01/89678 describes oil-rich emulsions having a ratio by weight of oily phase to aqueous phase of 0.7. However, the rate of release of the oil from these emulsions is not sufficient to obtain good effectiveness, as shown in the comparative examples given below.